If you are using newer Linux distributive, you might also need removing libz.so.1 file from JT Assistant distribution
(so that it will be used from system without conflicts with libpng library also installed in system).

Note that JT Assistant builds which currently available on download section are relatively outdated.
If you are looking for support of JT files within up-to-date state of TKJT library, it is better contacting Open CASCADE support services.https://www.opencascade.com/content/jt-import-export-sdk